What Causes Pressure In Abdomen Along With Blood Tissue In Stools?

I started drinking a protein shake four days ago and since then my stool is small oval/round in shape and there are not many of them at a time. There is also pink looking tissue attached and brown blood tinged mucous flakes. I don't have any stomach pain - just some pressure when I need to go to the bathroom. My anal opening is also a little sore, but I think that's just from wiping.